1. Field of the Invention
The present invention generally relates to an implanter, and more particularly, to a method and an apparatus for monitoring leakage current of a Faraday cup.
2. Description of the Prior Art
Ion implantation processes are widely used in semiconductor manufacture, for example, to implant wafers with various ions having desired energy. Ion implantation processes typically require uniform and consistent amounts of ions to be implanted into a semiconductor wafer.
FIG. 1A is a simplified diagram of a conventional implanter 100. The conventional implanter 100 includes an ion source 110, an analyzer magnet 120, and a Faraday cup 140. In the configuration, a chamber 130 is required to surround the target wafer 20 being implanted and to provide the space through which an ion beam 10 can travel. The ion source 110 is used to generate an ion beam 10 for outputting ions to be implanted into the target wafer 20. The ion beam 10 generated from the ion source 110 is analyzed by the analyzer magnet 120 before the required ions are implanted into the target wafer 20. As usual, the Faraday cup 140 is disposed behind the position of the target wafer 20, and is mounted on the chamber wall of the chamber 130. The chamber wall of the chamber 130 includes an opening 131 corresponding to the position of the Faraday cup 140, such that the ion beam 10 can be received by the Faraday cup 140 if the target wafer 20 is not located. Hence, the current meter 150 electrically coupled with the Faraday cup 140 can measure the ion beam 10 to be implanted into the target wafer 20. Moreover, the chamber wall of the chamber 130 is made of conductive material and coupled with the ground (i.e., electrically grounded). An electrical insulator 135 is disposed between the chamber 130 and the Faraday cup 140 such that the Faraday cup 140 is electrically insulated from the chamber 130 by the electrical insulator 135 whereby all received ion beam current can be measured by the current meter 150.
For a typical conventional ion implanter, a number of measuring and tuning steps must be conducted before ion implantation of wafers. For example, an ion beam current must be measured for precisely adjusting the parameters of the ion implantation, such that the ions practically implanted into the target wafer 20 has the desired energy and distribution. The ion beam current is the quantity of current of the ion beam 10 that impacts the target wafer 20. In the conventional implanter 100, the ion beam current is measured by the Faraday cup 140 before ion implantation of the target wafer 20.
FIG. 1B shows how an ion beam current is measured by the Faraday cup 140. The Faraday cup 140 essentially is a conductor shell for collecting the ion beam 10, and the current meter 150 electrically coupled with the Faraday cup 140 is connected to measure the ion beam current provided by the ion beam 10 collected by the Faraday cup 140. Clearly, with the exception of rare incidents, the ion beam current implanted into the target wafer 20 is equal to the ion beam current received by the Faraday cup 140 as a consequence of it being the same ion beam 10.
FIG. 1C shows the condition of the Faraday cup 140 after repeated measuring processes. Collision between the Faraday cup 140 and ions of the ion beam 10 results in formation of conductive atoms and/or conductive molecules on the surface of the Faraday cup 140 after the chargers of the ions are delivered to the current meter 150. Hence, following repeated measuring processes, conductive structures 30 tend to be formed on the surface of the Faraday cup 140. Moreover, these conductive structures 30 may be distributed randomly, especially after the “vacuum venting” process in which the gas pressure difference typically can induce irregular air flow (i.e., wind) inside of the chamber 130. Thereafter, the conductive structures 30 may electrically couple the Faraday cup 140 with the chamber wall of the chamber 130, such that a portion of the ion beam current received by the Faraday cup 140 will flow through the chamber 130 to the ground. Thereafter, the current measured by the current meter 150 is smaller than the real ion beam current received by the Faraday cup 140, such that the practical ions amount implanted into the target wafer 10 is higher than the excepted ions amount according to the measured ion beam current. As an unavoidable result, the target wafer 20 will be overdosed by the ion beam 10 mentioned above.
In order to prevent the target wafer 20 from being overdosed, these conductive structures 30 should be properly eliminated to avoid any current path formed by it. At least, the resistance between the Faraday cup 140 and the chamber 130 should be measured to properly adjust the measured ion beam current accordingly. As usual, these conductive structures 30 are eliminated or measured when the ion implanter is preventively maintained. Herein, the power of the ion implanter is turned off and the chamber 130 is opened (vacuum venting), such that the tools for eliminating/measuring these conductive structures 30 can be applied on these conductive structures 30 formed on the Faraday cup 140, especially on the surface of the Faraday cup 140 facing the analyzer magnet 120.
However, the cost of the preventive maintenance is high. Moreover, even if the conductive structures 30 is properly eliminated or measured during preventive maintenance, there is still a risk that the distribution of these conductive structures 30 may be changed during the period between preventive maintenances. In other words, the resistance between the Faraday cup 140 and the chamber 130 may be largely unknown between preventive maintenances, as a consequence of the incapability of monitoring real-time for the presence of conductive structures 30 between preventive maintenances. Therefore, the target wafer 20 may still be possibly overdosed by the ion beam 10.
Because of the disadvantages with the prior art such as mentioned above, a need exists to propose a novel method and an apparatus.